DISCING OF THE HAMILTON GROUP. ' 33 



narrow indentation on the anterior slope ; and in ventral valves in 

 similar condition, there is sometimes a lobed muscular impression just 

 anterior to the foramen. 



This species is very abundant in certain localities of the Genesee slate. The 

 oval-ovate forms rarely or never exceed nine-twentieths of an inch in length, by 

 eight-twentieths in the greatest width. The largest subcircular form obtained has 

 a length a little less than half an inch, with a width of full half an inch in its 

 greatest diameter, the posterior end being truncate. 



In form and proportions, this shell scarcely diifers from the D. media, but is 

 smaller than the generality of that species, the striae much finer and closer, and 

 it possesses the faint radiating folds which do not appear in that species. 



Geological formation and locality. In the Genesee slate, above the Hamilton 

 group : near Lodi and Bigatream point, Seneca county, N.Y. 



Disciiia truiicata ( n. s.). 



PLATES I & n. 



Shell ovate, the anterior end broader : valves depressed-convex. Dorsal 



valve with the apex near the posterior margin, and directed backwards ; 



posterior margin very abruptly rounded or truncate. Ventral valve 



with the apex submarginal ; foramen extending nearly or quite to the 



posterior margin, which is indented. 

 Surface marked by fine concentric striae and faint radiating undefined 



lines. 



This little species scarcely ever exceeds two-tenths of an inch in length ; while 

 the greatest breadth, which is anterior to the middle, is nearly the same. The 

 dorsal valve has the appearance of a Lingula, with flattened margin extending 

 beyond the beak : the ventral valve .shows a submarginal foramen. The muscular 

 impressions (which are faintly preserved ) differ from the ordinary Discina, but 

 are too obscure to afford means of separating it from the genus. 



In some specimens the posterior margin is abruptly rounded, while in others it 

 is straightly truncate. It may, perhaps, have been an elongated form of tliis shell 

 which is figured as Lingula concentrica in the Geological Report on the Fourth 

 District. 



Geological formation and locality. In the Genesee slate : near Lodi, and at 

 Bigstream point, Seneca county. 



